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dimensions update liveSelection Wizard & Design Advisor
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Separator Sizing Guide
A two-phase separator removes liquid droplets from a gas stream and separates gas from liquid by gravity settling and retention volume. It must satisfy two independent criteria - gas capacity and liquid retention - and the larger requirement governs the vessel size.
Gas capacity (Souders-Brown)
The allowable gas velocity is Vmax = K x sqrt((rhoL - rhoG)/rhoG). The actual gas volumetric flow at operating conditions divided by Vmax gives the required gas flow area, and hence a minimum diameter. The K-factor depends on the mist eliminator and is derated at high pressure.
Liquid retention
The liquid hold-up volume is the liquid flow rate times the retention time. In a horizontal vessel the liquid fills the lower portion over the effective length; in a vertical vessel it sets the liquid section height. Retention times are typically 1 to 3 minutes for oil and 3 to 10 minutes for water.
Horizontal vs vertical
Horizontal separators win when liquid retention dominates - high liquid load, slugging or three-phase duty. Vertical separators suit high gas-to-liquid ratios and tight plot space. The wizard recommends a type from the gas-liquid ratio and liquid loading.
Standards
Methodology references API 12J, the Souders-Brown method, the GPSA Engineering Data Book, API RP 14C/14E, and Arnold & Stewart Surface Production Operations. Mechanical design of the vessel follows ASME VIII Div.1. SepSizer is a screening and preliminary-sizing aid; confirm against the governing standards and a full process design before procurement or construction.
Key facts
- A two-phase separator is sized for both gas capacity and liquid retention; the larger requirement governs.
- Souders-Brown allowable gas velocity: Vmax = K x sqrt((rhoL - rhoG)/rhoG).
- K-factor by mist type: none 0.10-0.18, mesh 0.25-0.35, vane 0.30-0.50, cyclonic 0.40-0.60 ft/s; derate for pressure.
- Liquid volume = flow rate x (retention time + surge time); levels LLL/NLL/HLL set hold-up and surge.
- Typical L/D: 3-5 horizontal, 2-4 vertical. Inlet momentum rhoV2 limit ~1500 (bare) / ~9000 (diverter) lb/ft-s2.
- Standards: API 12J, GPSA, API RP 14C/14E; vessel mechanical design to ASME VIII. Screening-grade tool.
Frequently asked questions
What is a two-phase separator?
A two-phase separator is a pressure vessel that splits a gas-liquid stream into a gas phase and a single liquid phase by gravity settling and liquid retention. Gas exits the top through a mist extractor while liquid collects at the bottom under level control. It is sized by two criteria - gas capacity and liquid retention - and the larger one governs the vessel size.
How is a two-phase separator sized?
Sizing satisfies two independent requirements. Gas capacity keeps the gas velocity below the Souders-Brown allowable so droplets settle out, setting a minimum gas flow area. Liquid retention provides enough liquid volume to hold the liquid for the required time plus surge. The diameter and length are chosen so both are met, and the tool reports which one governs.
What is the Souders-Brown equation?
The Souders-Brown equation gives the maximum allowable gas velocity: Vmax = K x sqrt((rhoL - rhoG)/rhoG). K is an empirical capacity factor set by the mist eliminator and pressure, rhoL and rhoG are the liquid and gas densities. The required gas flow area is the actual gas volumetric flow at operating conditions divided by Vmax.
What is the K-factor in separator sizing?
The K-factor is an empirical capacity coefficient in ft/s. Typical values are 0.10-0.18 with no mist eliminator, 0.25-0.35 with a wire mesh pad, 0.30-0.50 with a vane pack and 0.40-0.60 with a cyclonic device. K must be derated as operating pressure rises and for foaming or fouling service - using a fixed K at high pressure is non-conservative.
How is retention time calculated?
Retention (residence) time is the liquid hold-up volume divided by the liquid volumetric flow rate. In sizing, the required liquid volume equals the flow rate times the target retention time. Typical targets are 1-3 minutes for oil and 3-10 minutes for water, with longer times for foaming, heavy or degassing service.
Horizontal or vertical separator - which is better?
Use a horizontal separator when liquid retention dominates: high liquid load, slugging, three-phase duty or low gas-liquid ratio. Use a vertical separator when gas capacity dominates: high gas-liquid ratio, limited plot space, solids handling or intermittent liquid. Horizontal vessels give more interface area per unit cost when liquid governs.
How do you calculate gas density for a separator?
Gas density follows the real-gas law rho = P x M / (Z x R x T), where P is absolute pressure, M is molecular weight (about 28.97 times the gas specific gravity), Z is the compressibility factor, R is the gas constant and T is absolute temperature. Z can be entered or estimated from a correlation such as Dranchuk-Abou-Kassem.
What is liquid hold-up volume?
Liquid hold-up volume is the volume of liquid the separator must contain to provide the required retention time at the design liquid flow rate. It occupies the lower part of a horizontal vessel or the bottom section of a vertical vessel and equals the liquid flow rate times the retention time.
What is surge volume?
Surge volume is liquid capacity provided above the normal liquid level (between NLL and HLL) to absorb slugs or short-term flow increases without tripping high-level alarms or carrying liquid into the gas outlet. It is provided in addition to retention volume and is sized from a slug volume or a surge time of about 1-3 minutes.
What are LLL, NLL and HLL?
LLL (low), NLL (normal) and HLL (high) liquid levels are control set-points in the liquid section. Retention volume sits between LLL and NLL, surge volume between NLL and HLL. They define alarm and trip points and set the liquid section height in a vertical vessel or the level fraction in a horizontal vessel.
What is the inlet momentum (rhoV2) check?
The inlet momentum check limits rho x V squared at the inlet nozzle, with rho the mixture density and V the nozzle velocity. Typical limits are about 1500 lb/ft-s2 with no inlet device and up to about 9000 with a diverter or vane inlet. Exceeding the limit shatters droplets into a fine mist that the gravity section cannot remove.
How is droplet removal verified?
Droplet removal is verified by comparing the terminal settling velocity of the design droplet, Vt = sqrt(4 g dp (rhoL - rhoG)/(3 Cd rhoG)), with the actual gas velocity. In a vertical vessel the gas velocity must stay below Vt; in a horizontal vessel the droplet must settle to the liquid surface within the gas residence time. Mist eliminators capture finer droplets.
What is the L/D ratio for separators?
The length-to-diameter ratio is a proportioning guide. Horizontal two-phase separators typically use L/D of about 3-5 (often 4); vertical separators use about 2-4. Values outside these ranges suggest the vessel type or the gas and retention assumptions should be revisited.
What is the governing criterion in separator sizing?
The governing criterion is whichever requirement - gas capacity or liquid retention - needs the larger vessel. If the liquid volume forces a bigger diameter or longer length than the gas requirement, liquid retention governs; otherwise gas capacity governs. Knowing which controls tells the engineer which variable to change.
What is a mist eliminator?
A mist eliminator is an internal device that captures fine droplets the gravity section cannot remove. Common types are wire mesh pads, vane packs and cyclonic devices. A more effective device allows a higher K-factor and a smaller vessel and removes smaller droplets, down to about 10 microns for mesh and cyclonic types.
What is API 12J?
API Specification 12J is the industry specification for oil and gas separators. It provides guidance on separator types, sizing principles, retention times and design considerations for two- and three-phase production separators, used together with the GPSA Engineering Data Book and references such as Arnold and Stewart for detailed sizing.
What retention time should I use for oil?
Typical oil retention times are 1 to 3 minutes for clean, non-foaming crude. Foaming, heavy or high-viscosity crude and degassing duties need longer - up to 5 minutes or more. The correct value depends on the crude, the gas disengagement required and downstream stability needs; confirm against project specifications.
How is a vertical separator's diameter sized?
A vertical separator's diameter is set by gas capacity: the full cross-section must be large enough that the upward gas velocity stays below the Souders-Brown allowable, so A = Q_gas / Vmax and D = sqrt(4A/pi). The height is then built from the liquid retention and surge section, gas disengagement, the mist extractor and the inlet zone.
What standards apply to two-phase separator design?
Separator sizing references API 12J, the Souders-Brown method, the GPSA Engineering Data Book, API RP 14C and 14E, and texts such as Arnold and Stewart Surface Production Operations and the Campbell Petroleum Engineering Handbook. The pressure-containing mechanical design of the vessel follows ASME Boiler and Pressure Vessel Code Section VIII.
Can a separator be sized on a phone or offline?
Yes. SepSizer runs entirely in the browser with no login and works offline, so separator sizing can be done on a phone, tablet or laptop in the field. All calculations, the drawing and the PDF, Excel, CSV and JSON exports are generated locally and nothing is uploaded.
Is a separator sizing calculator suitable for detailed design?
A separator sizing calculator is suitable for conceptual design, FEED screening, debottlenecking checks and training. Detailed design - internals selection, nozzle and momentum criteria, three-phase effects, level instrumentation and ASME VIII mechanical design - must be performed by a qualified engineer against the governing standards before procurement or construction.
Why does my separator come out very large?
A large diameter or length usually means liquid retention governs, the K-factor is low (or correctly derated at high pressure), the gas flow is high relative to the allowable velocity, or the retention and surge times are generous. Check which criterion governs, the K value used, and whether the chosen vessel type suits the gas-liquid ratio.